Malononitrile is a versatile intermediate used extensively in synthetic organic chemistry. In industry, it is largely used as building block for a variety of pharmaceuticals and pesticides, such as thiamine (vitamin B.sub.1), adenine, minoxidil (anti-hypertensive, Upjohn), thiopurinol (gout remedy), diuretic triamterene (Smith Kline), aminopeterin, bensulfuron-methyl (herbicide, Du Pont), etc. A variety of important methine dyes, in particular, aminoaryl cyanine based polyester dyes, characterized by high light fastness, are derived from malononitrile. Several important dyes belonging to this class are produced by major dye manufacturers such as Bayer, BASF, Ciba-Geigy, and Kodak. Malononitrile is also used in making tetracyanoquinodimethane (TCNQ), which forms with tetrathiafulvalene charge-transfer complex suitable for the production of conducting films in photocopiers and three-dimensional memories. On the other hand, o-chlorobenzylidene malononitrile (CS gas) is well-known tear gas with high safety factor.
Extensive literature search reveals that the preparation of malononitrile has largely relied upon dehydration of cyanoacetamide in a batch process utilizing a variety of dehydrating agents. It is also obtained in a continuous process involving gas phase reaction of cyanogen chloride and acetonitrile over a suitable catalyst at high temperature in a tube reactor.
Reference may be made to patents, such as U.S. Pat. No. 4,136,108, 1979; Neth. Appl. 80,04516, 1980; Swiss Appl. 68/6944, 1968; Ger. Often 3,006,492, 1981 wherein high conversion is attained in the transformation of acetonitrile to malonotirile. The drawbacks in the above described processes are the requirement of high reaction temperatures and low selectivity due to formation of maleic, succinic and fumeric acid by-products, which require efficient separation to obtain malononitrile in 60% yield (based on CNC1 or MeCN).
Reference may be made to the patent U.S. Pat. No. 5,959,136, 1999 wherein malononitrile is made from an isonitrile, optionally in the presence of a nitrile by using a similar procedure. The drawback in the above process is the requirement of high temperature for the activation of isonitrile, which isomerizes to nitrile, the reactive ingredient in the previous process.
Reference may be made to publication by E. M. Gal and A. T. Shulgin J. Amer. Chem. Soc. 73, 2938, 1951 wherein P.sub.2 O.sub.5 is employed as dehydrating agent. The drawback in the above described process is that the dehydrating agent is less effective in terms of selectivity and yield as compared to POC1.sub.3.
Reference may be made to publications by B. B. Corson, R. W. Scott and C. E. Vose Org. Synth. 10, 66, 1930; A. J. Fatidadi Synthesis 165, 1978 and patents, such as U.S. Pat No. 2,802,857, 1957; Brit. 1,163,397, 1969, wherein PC1.sub.5 is employed as dehydrating agents. The drawback in the above described process is that the dehydrating agent is less effective in terms of selectivity and yield as compared to POC1.sub.3.
Reference may be made to publications by Surrey, Org. Synth. 25, 63, 1945; J. Amer. Chem. Soc 65, 2471, 1943 wherein the preparation of malononitrile by elimination of water from cyanoacetamide, is extensively studied by employing POC1.sub.3 as dehydrating agent.
In POC1.sub.3 based procedures, a suspension of cyanoacetamide in dichloroethane is refluxed in the presence of POC1.sub.3. Nevertheless, this dehydration protocol possesses a severe process limitation when applied to large-scale synthesis due to formation of metastable phosphoric acid as side product, which, under the reaction conditions, tends to form hard polymeric crust along the reactor wall inhibiting uniform stirring and heating.
Moreover, a considerable portion of suspended starting material remains embedded within the coating causing substantial lowering of the yield. In order to avoid polymerisation of meta-phosphoric acid, inorganic salts are employed to convert it into its salt, which is precipitated from the reaction mixture. Several processes, employing alkali metal salts and alkaline earth metal salts, are disclosed.
Reference may be made to patents, for example, U.S. Pat. No. 2,389,217, 1945 and U.S. Pat. No. 2,799,697, 1957 wherein alkali metal salts are used for the purpose. The drawback in the above described process is that they involve consumption of a large quantity of inorganic salt, which is required in stoichiometric amount and can not be easily recovered due to the presence of tarry polymeric products. Thus, in the patent U.S. Pat. No. 2,389,217, 1945 1.00 Kg NaC1 per 1.26 Kg cyanoacetamide is recommended and no salt recovery is prescribed.
Reference may be made to publication by R. Malinowski and J. Legocki Organika 53, 1977 and U.S. Pat. No. 3,459,783, 1969 wherein alkaline earth metal salts are used. The drawback in the above described processes is that they involve consumption of a large quantity of inorganic salt, which is required in stoichiometric amount and can not be easily recovered due to the presence of tarry polymeric products.
However, in synthesizing malononitrile by utilizing above procedure, only a marginal improvement is noted since the formation of the polymeric product cannot be completely prevented. Therefore, notwithstanding the foregoing, the art has not heretofore taught or suggested a methodology that ensured process compatibility as well as offered environmentally clean technology.